The present invention relates to a fibrous wall material for cell structures useful in solar energy collectors and more particularly to solar energy collectors having fibrous walls that are substantially transparent to solar radiation and substantially impervious to long-wave radiation.
Solar energy collectors for converting the energy of solar radiation into heat as compared to solar cells, which utilize photoelectric effects, consist of a solar radiation absorber to absorb the radiation which is absorbed as completely as possible, and a suitable arrangement whereby heat is conducted from the absorber to the heat storage unit or directly to the device which utilizes the heat. The heat is generally carried away by a flowing medium (gas or liquid).
The absorber which is heated by the solar radiation, not only gives off its heat to the transporting medium, but also loses heat to the surroundings. Such undesired losses occur with both concentrating collectors and flat collectors.
With flat collectors, the side which is farthest from the incident solar radiation can be easily protected against heat losses. For example, conventional insulating materials, such as glass and rock wool or foam plastic materials, having a suitable thickness, provide good heat insulation at a low cost. It is more difficult to protect that side of the absorber which is exposed to the solar radiation against heat losses. Heat-insulating means which are arranged on this side of the absorber, must, in fact, satisfy the condition that the radiation is able to pass through the heat-insulating arrangements as far as possible, unhindered. Thus, the side of the absorber receiving the solar radiation should be substantially transparent for solar radiation.
Heat losses are caused by heat conduction, convection and radiation exchange. Steps which are taken for suppressing these heat losses frequently are only concerned with one of said forms of heat transfer, and sometimes, more than one of these forms simultaneously.
Heat losses of the solar collectors due to radiation exchange can be suppressed by various methods. Frequently, selectively reflecting layers or coatings are frequently used as absorbers. These layers absorb the solar radiation sufficiently well, but, on the other hand, only emit long-wave infrared to an insignificant degree. Coatings on transparent covering sheets or panes, which are transparent for the solar radiation, but are able to reflect long-wave infrared radiation, act in a similar manner. For example, if such a layer or coating is on the covering pane, on the side facing the absorber, the radiation emitted by the absorber is reflected on the layer and is again absorbed by it. Single or multiple covering panes, which are transparent for solar radiation, but absorb long-wave infrared, are not quite as effective as the measures which have been described above. If an intermediate space is subdivided by an additional pane, the heat transfer in this intermediate space is approximately halved due to radiation exchange.
Heat losses due to heat conduction and convection are closely related to one another with regard to solar energy collectors. The side of the absorber which faces the solar radiation is usually bounded by air. This layer of air conducts heat to the surroundings. It is not sufficient to make this gas layer so thick that the heat losses due to conduction become negligibly small. The convection, which likewise quickly increases, as the thickness of the gas layer increases, leads to the sum of the heat transfer fractions of conduction and convection being almost independent of the gas layer thickness, once a certain thickness is exceeded.
Thus, for example, with flat collectors having several covering panes which are transparent for solar radiaiton, the distance between the absorber and the pane disposed thereabove, or between two panes, and having a thickness of about 15 mm, has no influence on the heat-insulating properties of the arrangement. Any increase in the thickness of the gas layers results in an increase in convection.
One method frequently used for suppressing heat conduction and convection is to enclose the absorber in a vessel which permits the solar radiation to pass through to the absorber and is capable of being evacuated. Below a certain pressure, the convection is reliably suppressed. If the pressure is still further reduced, a point is then reached wherein a further decrease in the pressure reduces the heat conduction. Vessels which are capable of being evacuated must, however, be able to withstand the atmospheric pressure, and consequently this can only be achieved at great expense with flat collectors.
All methods so far known have been used in the widest possible range of application, both in connection with concentrating collectors and flat collectors.
Concentrating collectors with selectively reflecting absorber layers, selectively transmitting layers on the covering panes, or also both, are for example known. The enclosing vessels are often evacuated to a greater or lesser degree.
With regard to flat collectors, many arrangement with selectively reflecting absorber layers under single or multiple sheet or pane coverings have been tested.
Inherent in all of these combinations are disadvantages which cannot, in principle, be overcome by the measures which have been set forth herein.
Thus, at least one transparent covering is already required for keeping rain and dirt away from the absorber. Each additional covering, although desirable for heat insulation purposes, does however increase the absorption and reflection losses of the solar radiation in its passage through the covering to the absorber.
Selectively reflecting layers are expensive and usually present an absorption coefficient which is far removed from the optimum. Moreover, at relatively high temperatures, these layers are often unstable.
In order to avoid or minimize the aforementioned disadvantages, it has been proposed in the prior art to provide honeycomb-like structures between the absorber and the transparent cover sheets. If the shape and size and also the wall material are suitable chosen, then both the radiation exchange and the convection are reduced or almost completely suppressed.
The honeycomb walls generally stand perpendicular on the solar radiation absorber. HOTTEL.sup.1 previously showed that the radiation exchange between bottom and top of such honeycombs or cells is dependent upon the form or shape thereof and on the ratio between average diameter D and the height H of the cell. For cells having walls which absorb long-wave infrared, the radiation exchange -- as compared with unhindered exchange -- is reduced by a factor F.
______________________________________ Approximately the followings values apply: F = 0.52 0.36 0.27 0.22 0.19 0.10 ______________________________________ H/D = 1 2 3 4 5 10 ______________________________________ FNT .sup.1 HOTTELL, Mech.Eng. 52 (1930) 7, pages 699-704. HOTTELL, Am.Soc.Mech.Eng. Paper IS-55-6, Vol. 55, (1933) pages 39-49.
i.e., a cell structure of which the mean cell diameter amounts to only a tenth of the cell height, suppresses the heat transmission due to radiation by the factor 10, pre-supposing that the material of the cell wall absorbs long-wave infrared.
If the mean diameter of a single honeycomb is chosen small enough, then the convection is also suppressed. Depending on temperature difference and honeycomb height, it is possible to find a diameter below which the convection is completely suppressed..sup.2 For a temperature difference of 50.degree. C. between bottom and top of the honeycomb, it was possible to show that, below a cell diameter of 1 cm., the convection was completely suppressed..sup.3 FNT .sup.2 TABOR, Solar Energy, Vol. 11, pp. 549,552, Pergamon Press, 1969. FNT .sup.3 HOLLANDS, Solar Energy, Vol. 9, No. 3, 1965, pp. 159-164.
In the foregoing, only the properties of such cell structures have been described, which properties are of importance for the heat-insulating behavior thereof. In addition to having the properties as described, the honeycombs must, above all, allow the solar radiation to reach the absorber. The material of the cell wall must therefore be transparent or highly reflecting for solar radiation. In both cases, the solar radiation is able to reach the base of the cell, said base being the absorber. If the collector is caused to follow the position of the sun, then it is also possible to manage with thin honeycomb wall materials which are impervious to solar radiation. The honeycomb walls then only have to stand parallel to the incident radiation.
There is another condition which has to be set for the cell walls. They are to be as thin as possible, so that the heat conduction in the cell walls is small and does not make a considerable contribution to the heat losses of the collector due to conduction in the material of the cell wall.
For the first time, cell structures have been used by Russian scientists for solar energy collectors..sup.4 They used specially treated paper for the manufacture of the honeycomb structures. A new impetus resulted from the use of honeycomb structures by FRANCIA..sup.5 He used bunched glass tubes for producing high temperatures. Foils of synthetic plastics likewise initially seemed to be a very suitable material for the honeycomb walls and experiments have also been carried out with these. PERROT et al.sup.6 have experimented with honeycombs of synthetic plastic foils. The results did not come up to expectations, since thin foils of synthetic plastics are partially transparent for the long-wave infrared radiation. BUCHBERG et al.sup.7 used paper as the wall material, said paper having been vapor-coated with aluminum, so that the solar radiation was reflected down to the absorber. The surface of the aluminum was coated with thick lacquer coatings (transparent to solar radiation), the purpose of these coatings being to provide for the long-wave infrared being absorbed in them. FNT .sup.4 V. B. VEINBERG, Optics in Equipment for the Utilization of Solar Energy, State Publishing House of Defense Ministry, Moscow (1959(, (Translated by U.S. Dept. of Army Intelligence, Translation No. 44787, or USAEC Translation AEC-tr-4471). FNT .sup.5 G. FRANCIA, Paper E/Conf. 35/5/71. U.N. Conf. on New Sources of Energy, Rome (1961). FNT .sup.6 PERROT, Solar Energy, g (1967) Vol. 11, no. 1, pp. 34-40. FNT .sup.7 BUCHBERG, Solar Energy, Vol. 13, pp. 193-221, Pergamon Press, 1971.
If the prior experiments with solar energy collectors having honeycomb structures for suppressing the heat losses are summarized, the conclusion is reached that it would have been possible to construct very good collectors in accordance with this principle, if only suitable materials were available for the honeycomb structures.
The satisfactory materials are those which are transparent for solar radiation. If such wall materials have an optically good surface, i.e., if they only disperse the radiation to a very slight degree, and if the material has low absorption power, then a very high percentage reaches the bottom of the cells, that is to say, reaches the absorber. Wall materials which are not transparent for solar radiation are basically less suitable, since there are no simple coatings which reflect solar radiation free from heat loss. A portion of the solar radiation accordingly does not reach the absorber.
It is just as difficult and unsatisfactory as regards the power of absorption for long-wave infrared, which is necessary so that the radiation exchange is reduced.
If thin foils of plastic materials are used, then a considerable proportion of the long-wave infrared radiation is allowed to pass through. It would be possible to use thicker foils, in order to increase the absorption. It is only a slight improvement which can be obtained in this way because of the typical band structure of the infrared transmission spectra of organic polymers. In spectral regions of high transmissions, it is necessary to have foils of such great thickness in order to noticeably restrict the transmission actually on the said foils, and the cost of the foil would then be an obstacle.
For example, a Hostaphan foil with a thickness of 75 .mu.m still allows the passage of about 20% of the radiation of a black body of 350.degree. C. Copolymers having a composition which has been selected so that spectral regions of great transmissivity of the one polymer are covered by the absorption bands of the other polymer are of some assistance. Lacquered or lined foils are likewise possible. An additional disadvantage is that the plastic materials have to be exceptionally stable to various radiation effects. As more different plastic materials contributing to the fabrication of the foil, the more difficult it is to satisfy the stability conditions in addition to the properties which have already been discussed hereinbefore. A need therefore exists to eliminate or minimize the aforementioned difficulties and disadvantages.